The characterization of fluid samples, such as biological samples, is increasing in importance. Further, it is known to investigate a sample placed on a first surface of a sample stage element, which sample stage element presents with first and second, typically, but not necessarily substantially parallel surfaces, by utilizing an electromagnetic beam applied from said first surface side of said sample stage element such that said beam reflects from said sample into a detector. It is further known to independently investigate a sample placed on a sample stage element first surface utilizing an electromagnetic beam applied from a second, oppositely facing surface side of said sample stage element such that said beam reflects from the sample into a detector. Of course the sample stage element must be transparent to said electromagnetic radiation applied from the second surface side thereof in order to access the sample. Further, it is to be understood that electromagnetic radiation can be of any functional wavelength, either monochromatic, (ie. laser source), or spectroscopic.
The primary motivation for the disclosed invention is found in a need to do more definitive assays and analysis in areas such as:                antibody/antigen interactions;        microbiology (eg. viruses, toxins etc.);        physiological (eg. hormones);        drugs (therapeutic and illegal).In addition, the present invention finds application in fundamental science where, for instance, bonding mechanisms and attachment rates for proteins and/or DNA to surfaces and other biomaterials are of interest.        
The application of Spectroscopic Ellipsometry (SE) to biologics provides utility because reflectance from Bio-films on opaque substrates is difficult to detect where intensity changes are small. In addition Surface Plasmon Resonance (SPR), while sensitive, has a limited spectral range and can be applied only to limited types of substrate materials and layer thicknesses.
It is noted that a suitable system for investigating biologics must be relatively immune to such as temperature sensitive birefringence of electromagnetic wavelength windows, which requires careful design and mounting. In addition, temperature sensitivity of reagents and reactions and reagent concentration sensitivity can enter artifacts into results, hence a suitable system for investigating biologics must provide means to minimize random effects therein. A robust system and method therefore should provide compensation capability, at least to compensate the identified birefringence, during data in analysis.
Continuing, while the herein disclosed invention can be used in any material system investigation system such as Polarimeter, Reflectometer, Spectrophotometer and the like Systems, an important application is with Ellipsometer Systems, whether monochromatic or spectroscopic. It should therefore be understood that Ellipsometry involves acquisition of sample system characterizing data at single or multiple Wavelengths, and at one or more Angle(s)-of-Incidence (AOI) of a Beam of Electromagnetic Radiation to a surface of the sample system. Ellipsometry is generally well described in a great many publication, one such publication being a review paper by Collins, titled “Automatic Rotating Element Ellipsometers: Calibration, Operation and Real-Time Applications”, Rev. Sci. Instrum., 61(8) (1990).
A typical goal in ellipsometry is to obtain, for each wavelength in, and angle of incidence of said beam of electromagnetic radiation caused to interact with a sample system, sample system characterizing PSI and DELTA values, where PSI is related to a change in a ratio of magnitudes of orthogonal components rp/rs in said beam of electromagnetic radiation, and wherein DELTA is related to a phase shift entered between said orthogonal components rp and rs, caused by interaction with said sample system. This is expressed by:TAN(ψ)ei(Δ)=rs/rp.(Note the availability of the phase DELTA (Δ) data is a distinguishing factor between ellipsometry and reflectometry).
Continuing, Ellipsometer Systems generally include a source of a beam of electromagnetic radiation, a Polarizer, which serves to impose a state of polarization on a beam of electromagnetic radiation, a Stage for supporting a sample system, and an Analyzer which serves to select a polarization state in a beam of electromagnetic radiation after it has interacted with a material system, and passed it to a Detector System for analysis therein. As well, one or more Compensator(s) can be present and serve to affect a phase angle between orthogonal components of a polarized beam of electromagnetic radiation. A number of types of ellipsometer systems exist, such as those which include rotating elements and those which include modulation elements. Those including rotating elements include Rotating Polarizer (RP), Rotating Analyzer (RA) and Rotating Compensator (RC). A preferred embodiment is a Rotating Compensator Ellipsometer System because, it is noted, Rotating Compensator Ellipsometer Systems do not demonstrate “Dead-Spots” where obtaining data is difficult. They can read PSI and DELTA of a Material System over a full Range of Degrees with the only limitation being that if PSI becomes essentially zero (0.0), DELTA can not then be determined as there is not sufficient PSI Polar Vector Length to form the angle between the PSI Vector and an “X” axis. In comparison, Rotating Analyzer and Rotating Polarizer Ellipsometers have “Dead Spots” at DELTA's near 0.0 or 180 Degrees and Modulation Element Ellipsometers also have “Dead Spots” at PSI near 45 Degrees). The utility of Rotating Compensator Ellipsometer Systems should then be apparent. Another benefit provided by fixed Polarizer (P) and Analyzer (A) positions is that polarization state sensitivity to input and output optics during data acquisition is essentially non-existent. This enables relatively easy use of optic fibers, mirrors, lenses etc. for input/output.
Further, it is to be understood that causing a polarized beam of electromagnetic radiation to interact with a sample system generally causes change in the ratio of the intensities of orthogonal components thereof and/or the phase angle between said orthogonal components. The same is generally true for interaction between any system component and a polarized beam of electromagnetic radiation. In recognition of the need to isolate the effects of an investigated sample system from those caused by interaction between a beam of electromagnetic radiation and system components other than said sample system, (to enable accurate characterization of a sample system per se.), this Specification incorporates by reference the regression procedure of U.S. Pat. No. 5,872,630 to Johs et al. in that it describes simultaneous evaluation of sample characterizing parameters such as PSI and DELTA, as well system characterizing parameters, and this Specification also incorporates by reference the Vacuum Chamber Window Correction methodology of U.S. Pat. No. 6,034,777 to Johs et al. to account for phase shifts entered between orthogonal components of a beam of electromagnetic radiation, by disclosed invention system windows and/or beam entry elements.
A Published Patent Application of which the applicants are aware is U.S. 2002/0024668 by Stehle et al. This application discloses the use of two electromagentic beams applied orthogonally to a sample, and one electromagentic beam applied normally thereto through effective windows which are oriented parallel to the surface of the sample.
Other patents of which the Inventor is aware include U.S. Pat. No. 5,757,494 to Green et al., in which is taught a method for extending the range of Rotating Analyzer/Polarizer ellipsometer systems to allow measurement of DELTA'S near zero (0.0) and one-hundred-eighty (180) degrees. Said patent describes the presence of a window-like variable bi-refringent components which is added to a Rotating Analyzer/Polarizer ellipsometer system, and the application thereof during data acquisition, to enable the identified capability.
A patent to Thompson et al. U.S. Pat. No. 5,706,212 teaches a mathematical regression based double Fourier series ellipsometer calibration procedure for application, primarily, in calibrating ellipsometers system utilized in infrared wavelength range. Bi-refringent window-like compensators are described as present in the system thereof, and discussion of correlation of retardations entered by sequentially adjacent elements which do not rotate with respect to one another during data acquisition is described therein.
A patent to Woollam et al, U.S. Pat. No. 5,582,646 is disclosed as it describes obtaining ellipsometric data through windows in a vacuum chamber, utilizing other than a Brewster Angle of Incidence.
Patent to Woollam et al, U.S. Pat. No. 5,373,359, patent to Johs et al. U.S. Pat. No. 5,666,201 and patent to Green et al., U.S. Pat. No. 5,521,706, and patent to Johs et al., U.S. Pat. No. 5,504,582 are disclosed for general information as they pertain to Rotating Analyzer ellipsometer systems.
Patent to Bernoux et al., U.S. Pat. No. 5,329,357 is identified as it describes the use of optical fibers as input and output means in an ellipsometer system.
U.S. Pat. No. 5,991,048 To Karlson et al. describes a system for practicing Surface Plasmon Resonance in which a light pipe arrangement is present upon which can be situated a flow cell. Sample entered to the flow cell becomes situated on the upper surface of the light pipe and light entered to the light pipe interacts with it from below, then exists and enters a multi-element detector at various angles.
U.S. Pat. No. 6,316,274 B1 to Herron et al. describes a single light source system for practicing multi-analyte homogeneous flouro-immunoassays, via detecting of reflected and transmitted beams.
U.S. Pat. No. 5,313,264 to Ivarsson et al. describes a single light source system in which a light beam accesses a sample via a prism, (which can be semicircular in shape), and reflects into a detector.
U.S. Pat. No. 4,159,874 to Dearth et al. describes another single light source system which includes upper and lower sensors.
U.S. Pat. No. 6,200,814 B1 to Malmquist et al. describes a method and system for providing laminar flow over one or more discrete sensing areas.
U.S. Pat. No. 4,076,420 to De Maeyer et al. describes a system for investigating fast chemical reactions by optical detection of, for instance, absorbtion or fluorescence or scattered light, including detection of polarized light.
Patents identified during the preparation and prosecution of. Pending application Ser. No. 09/756,515, from which this application is a CIP are:    U.S. Pat. No. 5,486,701 to Norton et al.;    U.S. Pat. No. 5,900,633 to Solomon et al.;    U.S. Pat. No. 4,807,994 to Felch et al.;    U.S. Pat. No. 4,472,633 to Motooka;    U.S. Pat. No. 6,049,220 to Borden et al.
Scientific Articles are also identified as follows:
An article by Johs, titled “Regression Calibration Method For Rotating Element Ellipsometers”, which appeared in Thin Film Solids, Vol. 234 in 1993 is also identified as it describes an approach to ellipsometer calibration.
Another paper, by Straaher et al., titled “The Influence of Cell Window Imperfections on the Calibration and Measured Data of Two Types of Rotating Analyzer Ellipsometers”, Surface Sci., North Holland, 96, (1980), describes a graphical method for determining a plane of incidence in the presence of windows with small retardation.
An article by Collins titled “Automated Rotating Element Ellipsometers: Calibration, Operation, and Real-Time Applications”, Rev. Sci. Instrum. 61(8), August 1990 is disclosed for the general insight to ellipsometer systems it provides.
An article by Kleim et al. titled “Systematic Errors in Rotating-Compensator Ellipsometry” published in J. Opt. Soc. Am./Vol. 11, No. 9, September 1994 is identified as it describes calibration of rotating compensator ellipsometers.
An Article by An and Collins titled “Waveform Analysis With Optical Multichannel Detectors: Applications for Rapid-Scan Spectroscopic Ellipsometer”, Rev. Sci. Instrum., 62 (8), August 1991 is also identified as it discusses effects such as Detection System Error Characterization, Stray Light, Image Persistence etc., and calibration thereof.
A paper which is co-authored by the inventor herein is titled “In Situ Multi-Wavelength Ellipsometric Control of Thickness and Composition of Bragg Reflector Structures”, by Herzinger, Johs, Reich, Carpenter & Van Hove, Mat. Res. Soc. Symp. Proc., Vol.406, (1996) is also disclosed.
A paper by Nijs & Silfhout, titled “Systematic and Random Errors in Rotating-Analyzer Ellipsometry”, J. Opt. Soc. Am. A., Vol. 5, No. 6, (June 1988) is also identified.
An article by Jellison Jr. titled “Data Analysis for Spectroscopic Ellipsometry”, Thin Film Solids, 234, (1993) is also disclosed.                Papers of interest in the area by Azzam & Bashara;        “Unified Analysis of Ellipsometry Errors Due to Imperfect Components Cell-Window Birefringence, and Incorrect Azimuth Angles”, J. of the Opt. Soc. Am., Vol 61, No. 5, (May 1971);        “Analysis of Systematic Errors in Rotating-Analyzer Ellipsometers”, J. of the Opt. Soc. Am., Vol. 64, No. 11, (November 1974).        
An unpublished article by Poksinski et al. titled “Total Internal Reflection Ellipsometry, describes application of total internal reflection to investigate protein using ellipsometric techniques.
It is also mentioned that a book by Azzam and Bashara titled “Ellipsometry and Polarized light” North-Holland, 1977 is disclosed and incorporated herein by reference for general theory, as is a book which is authority regarding mathematical regression, (ie. a book titled Numerical Recipes in “C”, 1988, Cambridge University Press.
Continuing, to obtain valid data from an Ellipsometer, it is necessary to calibrate it. For insight, as generally described in the 630 patent with focus on a method of calibrating a spectroscopic rotating compensator material system investigation system, a generalized method of calibrating a material system investigation system can comprise the steps of:                a. providing a material system investigation system comprising a source of a polychromatic beam of electromagnetic radiation, a polarizer, a stage for supporting a material system, an analyzer, a dispersive optics and at least one detector system which contains a multiplicity of detector elements, said material system investigation system optionally comprising at least one compensator(s) positioned at a location selected from the group consisting of:                    before said stage for supporting a material system, and            after said stage for supporting a material system, and            both before and after said stage for supporting a material system;such that when said material system investigation system is used to investigate a material system present on said stage for supporting a material system, at least one of said analyzer or polarizer or at least one of said at least one compensator(s) is/are caused to continuously rotate while a polychromatic beam of electromagnetic radiation produced by said source of a polychromatic beam of electromagnetic radiation is caused to pass through said polarizer and said compensator(s), said polychromatic beam of electromagnetic radiation being also caused to interact with said material system, pass through said analyzer and interact with said dispersive optics such that a multiplicity of essentially single wavelengths are caused to simultaneously enter a corresponding multiplicity of detector elements in said at least one detector system;                        b. in conjunction with other steps, developing a mathematical model of said material system investigation system which comprises as calibration parameter variables such as polarizer azimuthal angle orientation, present material system PSI, present material system DELTA, compensator azimuthal angle orientation(s), matrix components of said compensator(s), analyzer azimuthal angle orientation, and angle of incidence changing system representations, which mathematical model is effectively a transfer function which enables calculation of electromagnetic beam intensity as a function of wavelength detected by a detector element, given intensity as a function of wavelength provided by said source of a polychromatic beam of electromagnetic radiation;        c. causing a polychromatic beam of electromagnetic radiation produced by said source of a polychromatic beam of electromagnetic radiation, to pass through said polarizer, interact with a material system caused to be in the path thereof, pass through said analyzer, and interact with said dispersive optics such that a multiplicity of essentially single wavelengths are caused to simultaneously enter a corresponding multiplicity of detector elements in said at least one detector system, with said polychromatic beam of electromagnetic radiation also being caused to pass through present compensator(s);        d. obtaining an at least two dimensional data set of intensity values vs. wavelength and a parameter selected from the group consisting of:                    angle-of-incidence of said polychromatic beam of electromagnetic radiation with respect to a present material system, and            azimuthal angle rotation of one element selected from the group consisting of:                            said polarizer; and                said analyzer;                at least one of said at least one compensator(s);                                    while at least one selection from the group consisting of                            said polarizer; and                said analyzer;                at least one of said at least one compensator(s);is caused to continuously rotate;                                                e. performing a mathematical regression of said mathematical model onto said at least two dimensional data set, thereby evaluating calibration parameters in said mathematical model;said regression based calibration procedure evaluated calibration parameters serving to compensate said mathematical model for non-achromatic characteristics and non-idealities of said compensator(s), and for azimuthal angles of said polarizer, analyzer and compensator(s).        
Said method of calibrating a material system investigation system can further comprise including calibration parameters for detector element image persistence and read-out non-idealities in the mathematical model, and further evaluating said calibration parameters for detector element image persistence and read-out non-idealities in said regression procedure.
Said method of calibrating a material system investigation system can include, in the step of developing a calibration parameter containing mathematical model of said spectroscopic rotating compensator ellipsometer system, the steps of providing a matrix representation of each of said polarizer, present material system, said compensator(s), and said analyzer etc., and determining a mathematical transfer function relating electromagnetic beam intensity out to intensity in, as a function of wavelength, by multiplication of said matrices.
Said method of calibrating a material system investigation system can further comprise the step of parameterizing calibration parameters by representing variation as a function of a member of the group consisting of: (wavelength, angle-of-incidence of said polychromatic beam of electromagnetic radiation with respect to a present material system, and azimuthal angle orientation of one element selected from the group consisting of: (said polarizer and said analyzer)), by a parameter containing mathematical equation, said parameters being evaluated during said mathematical regression.
Said method of calibrating a material system investigation system can preferably specifically include selecting calibration parameters which are parameterized, (such as polarizer azimuthal angle orientation, compensator azimuthal angle orientation(s), matrix components of said compensator(s), and analyzer azimuthal angle orientation), each as a function of wavelength.
Said method of calibrating a spectroscopic rotating compensator material system investigation system can involve using a material system which is selected from the group consisting of: (open atmosphere with the spectroscopic rotating compensator material system investigation system being oriented in a “straight-through” configuration, and other than open atmosphere with the spectroscopic rotating compensator material system investigation system being oriented in a “material-present” configuration).
Continuing, it should also be appreciated that when ellipsometer system components/elements are sequentially located adjacent to one another and are stationary with respect to one another, an ellipsometer “sees” the sum total thereof as a composite single element. For instance, if a sample system is present between two elements of a present invention electromagnetic beam intercepting angle-of-incidence changing system, an ellipsometric investigation will provide a PSI and DELTA of the composite thereof. This is clearly not what is desired. In view of this it is presented that the methodology described in the 777 patent, which is focused in application to correcting for phase shifts between orthogonal components of a polarized electromagnetic beam caused by its passing through vacuum chamber input and output windows, can be applied to compensate the effects of the presence of an invention electromagnetic beam intercepting angle-of-incidence changing system, as well. As insight to what is taught in the 777 patent consider that in-situ application of ellipsometry to investigation of a sample system present in a vacuum chamber presents a challenge to users of ellipsometer systems in the form of providing a mathematical model for each of said input and output windows, and providing a method by which the effects of said windows can be separated from the effects of an investigated sample system. (Like a disclosed invention system, input and output windows in a vacuum chamber are structurally positioned by said vacuum chamber and are not rotatable with respect to a sample system present in said vacuum chamber in use, thus preventing breaking correlation between parameters in equations for sequentially adjacent input and output windows and an investigated sample system by an element rotation technique). While correlation of parameters in mathematical equations which describe the effects of groupings of elements, (such as a compensator and an optional element(s)), can be tolerable, correlation between parameters in the mathematical model of an investigated sample system and other elements in the ellipsometer system must be broken to allow obtaining accurate sample system representing PSI and DELTA values, emphasis added. That is to say that correlation between parameters in a equations in a mathematical model which describe the effects of a stationary compensator and a sequentially next window element, (eg. correlation between effects of elements c. and d. or between f. and g. identified above), on a beam of electromagnetic radiation might be tolerated to the extent that said correlation does not influence determination of sample system describing PSI and DELTA values, but the correlation between parameters in equations which describe the effects of ellipsometer system components (eg. a., b., c., d., f., g., h. and i. identified above), and equations which describe the effects of a present sample system (eg. element e. above), absolutely must be broken to allow the ellipsometer system to provide accurate PSI and DELTA values for said sample system.
The 777 patent describes a method of accurately evaluating parameters in parameterized equations in a mathematical model of a system of spatially separated input and output windows, as applied in an ellipsometry or polarimeter setting which can be applied to the disclosed invention. Said 777 patent parameterized equations enable, when parameters therein are properly evaluated, independent calculation of retardation entered by each of said input window and said output window between orthogonal components of a beam of electromagnetic radiation caused to pass through said input and output windows. (It is to be understood that at least one of said input and output windows is bi-refringent).
While not independently establishing Patentability, a method of accurately evaluating parameters in parameterized equations in a mathematical model of a system of spatially separated input and output windows, said parameterized equations enabling, when parameters therein are properly evaluated, independent calculation of retardation entered by each of said input output windows between orthogonal components of a beam of electromagnetic radiation caused to pass through said input and output windows, at least one of said input and output windows being bi-refringent, comprises, in a functional order, the steps of:                a. providing spatially separated input and output windows, at least one of which input and output windows demonstrates birefringence when a beam of electromagnetic radiation is caused to pass therethrough, and further providing a means for supporting a sample system positioned between said input and output windows;        b. positioning an ellipsometer system source of electromagnetic radiation and an ellipsometer system detector system such that in use a beam of electromagnetic radiation provided by said source of electromagnetic radiation is caused to pass through said input window, interact with a sample, in a plane of incidence thereto, and exit through said output window and enter said detector system;        c. providing a sample to said means for supporting a sample system, the composition of said sample system being sufficiently well known so that retardance entered thereby to a polarized beam of electromagnetic radiation of a given wavelength, which is caused to interact with said sample system in a plane of incidence thereto, can be accurately modeled mathematically by a parameterized equation which, when parameters therein are properly evaluated, allows calculation of retardance entered thereby between orthogonal components of a beam of electromagnetic radiation caused to interact therewith in a plane of incidence thereto;        d. in conjunction with other steps, providing a mathematical model for said ellipsometer system and said input and output windows, which comprises separate parameterized equations for independently calculating retardance entered between orthogonal components of a beam of electromagnetic radiation caused to pass through each of said input and output windows; such that where parameters in said mathematical model are properly evaluated, retardance entered between orthogonal components of a beam of electromagnetic radiation which passes through each of said input and output window, and further interacts with said sample system in a plane of incidence thereto can be independently calculated from said parameterized equations;        e. obtaining a spectroscopic set of ellipsometric data with said parameterizable sample system present on the means for supporting a sample system, utilizing a beam of electromagnetic radiation provided by said source of electromagnetic radiation, said beam of electromagnetic radiation being caused to pass through said input window, interact with said parameterizable sample system in a plane of incidence thereto, and exit through said output window;        f. by utilizing said mathematical model provided in step d. and said spectroscopic set of ellipsometric data obtained in step e., simultaneously evaluating parameters in said mathematical model parameterized equations for independently calculating retardance entered between orthogonal components in a beam of electromagnetic radiation caused to pass through said input window, interact with said sample in a plane of incidence thereto, and exit through said output window;to the end that application of said parameterized equations for each of said input and output window and sample for which values of parameters therein have been determined in step f., enables independent calculation of retardance entered between orthogonal components of a beam of electromagnetic radiation by each of said input and output windows, and said sample system, at given wavelengths in said spectroscopic set of ellipsometric data, said calculated retardance values for each of said input window, output window and sample system being essentially uncorrelated.        
Said method preferably, in step f., involves simultaneous evaluation of parameters in said mathematical model parameterized equations for said parameterizable sample, and for said input and output windows, and is achieved by a square error reducing mathematical curve fitting procedure.
Further, said method, in step d., involves provision of a mathematical model for said ellipsometer system and said input and output windows parameterizable sample, can involve, for each of said input and output windows, providing separate parameterized mathematical model equations for retardance entered to each of said two orthogonal components of a beam of electromagnetic radiation caused to pass through said input and output windows; at least one of said orthogonal components for each of said input and output electromagnetic beam intercepting angle-of-incidence changing systems being directed out of the plane of incidence of said electromagnetic beam onto said parameterizable sample system; such that calculation of retardation entered between orthogonal components of said beam of electromagnetic radiation, by said input window is provided by comparison of retardance entered to each of said orthogonal components for said input window, and such that calculation of retardation entered between orthogonal components of said beam of electromagnetic radiation, by said output window is provided by comparison of retardance entered to each of said orthogonal components for said output window.
Said method, in step f., provides for simultaneous evaluation of parameters in said mathematical model parameterized equations for said parameterizable sample system, and for said input and output windows, is preferably achieved by a square error reducing mathematical curve fitting procedure.
Said method, in step b., provides for positioning of an ellipsometer system source of electromagnetic radiation and an ellipsometer system detector system and typically includes positioning a polarizer between said source of electromagnetic radiation and said input window, and the positioning of an analyzer between said output window and said detector system, and the step e. obtaining of a spectroscopic set of ellipsometric data typically involves obtaining data at a plurality of settings of at least one component selected from the group consisting of:                said analyzer; and        said polarizer.        
Said method, in the step of providing mathematical model parameterized equations for enabling independent calculation of retardance entered by said input and said output window and a sample system between orthogonal components of a beam of electromagnetic radiation, can involve use of parameterized equations having a form selected from the group consisting of:ret(λ)=K1+(K2/λ)+(K3/λ);ret(λ)=(K1/λ)ret(λ)=(K1/λ)*(1+(K2/λ))ret(λ)=(K1/λ)*(1+(K2/λ)+(K3/λ)).
As the present application is a CIP from Pending application Ser. No. 09/756,515, it is also disclosed that the basic method disclosed therein enables quantifying thickness and impurity profile defining parameters in impurity profile containing membranes, via a method comprising the steps of providing an ellipsometer system, and sequentially or simultaneosly obtaining ellipsometric data sets from both first and second sides of an impurity profile containing membrane, and providing a mathematical model of said impurity profile defining parameters comprising membrane thickness and impurity profile defining parameters, then performing a mathematical regression of said mathematical model onto data obtained from said impurity profile containing membrane by a selection from the group consisting of:                utilizing the data sets obtained from front and back of the thin membrane simultaneously;        utilizing the data sets obtained from front and back of the thin membrane independently; and        utilizing the data sets obtained from front and back of the thin membrane both independently and simultaneously;to evaluate said membrane thickness and impurity profile defining parameters. The concept of obtaining data from both sides of a sample, simultaneously or sequentially, and utilizing said data obtained simultaneously or independently to characterize the sample is thus established by said 515 application, and as regards the present invention, the “impurity profile defining parameters” in the 515 application can be considered analogically similar to the fluid sample atop a two sided stage in the present application.        
Even in view of the prior art, there is identified a remaining need for a system which allows essentially simultaneous investigation of a sample, particularly a biological fluid sample, with two wavelengths, one wavelength being caused to approach from one side of the sample, and the second wavelength being caused to approach said sample from the second side thereof.